Inflammation of the piriformis muscle, a small but important hip rotator in the buttock. Symptoms: Soreness in the buttock (especially where the top of your jeans pocket would be) often accompanied by pain or pins and needles along back of the thigh. Common causes: An excessively tight piriformis, tight inner thigh muscles and weak outer thigh (hip) muscles. Overstriding (strides that are too long), running on cambered roads or excessive downhill running. What to do: Use a tennis ball to massage the sore area and apply ice. Stretch and strengthen the appropriate muscles.